This invention is in the field of wireless communications, and is more specifically directed to signal modulation in such communications.
Modern advanced mobile computing devices and wireless telephone handsets are evolving from the so-called second generation (2G) technologies for wireless communications toward the capability of providing the so-called third generation (3G) wireless services. These 3G services are expected to extend current second generation voice and data services, and to include new very high bandwidth entertainment services including video and CD quality audio, interactive messaging including video and graphics, videoconferencing, video streaming, and remote control and monitoring services. These high-bandwidth services and applications of course place significant pressure on the wireless hardware, both on the transmission and receiving sides.
In addition to the differences in communications technologies between 2G and 3G wireless generations, each of the so-called generations are currently realized according to multiple broadband communications standards. Indeed, the 3G communications standard itself (“IMT-2000”) defines a family of radio interfaces that are suitable for a wide range of environments. Furthermore, some wireless communications standards are extensions of 2G modulation techniques, extending the data rates of 2G standards toward the levels required for 3G communications. To further complicate this field, different regions of the world have gravitated toward different wireless communications technologies. As a result, multiple wireless communication modes are now being used, and will likely continue to be used even into the 3G class of services.
Examples of 2G communications standards include the Global System for Mobile (GSM) and General Packet Radio System (GPRS) standards. Extensions of these approaches that are evolving toward 3G services include Enhanced Data rates for GSM Evolution (EDGE), which involves an eight-level phase shift keying (8-PSK) modulation at 200 kHz channel spacing, and CDMA 2000, which is an evolution from the TIA IS-95 code division multiple access (CDMA) standard. 3G cellular techniques are expected to include the Universal Mobile Telecommunications System (UMTS) and UTRA standards. In addition to these longer range techniques, the so-called Bluetooth short-distance wireless technology is also becoming popular in the art, for communication of wireless peripheral devices and systems with computer workstations. It is contemplated that these and other wireless standards will be implemented in the industry.
This multiplicity of communications technologies makes multi-mode transceiver circuitry desirable in the art. By definition, multi-mode transceiver circuitry is capable, on its transmitter side, of receiving a baseband input signal and modulating this signal according to multiple standards or communications technologies. Similarly, multi-mode receiving circuitry may receive a signal according to any one of the multiple technologies.
Multi-mode integrated transceiver circuits are now being developed to provide single-chip (or reduced chip count) circuitry for carrying out multi-mode transmission and receipt. These integrated circuits of course themselves provide multi-mode capability, and are well-suited for use in multi-mode wireless handsets and mobile devices. In addition, the multi-mode integrated circuits provide the integrated circuit manufacturer with an efficient way of manufacturing and controlling inventory of transceiver circuits suitable for use in equipment compliant with any one of the standards.
FIG. 1 illustrates the construction of a conventional multi-mode transmitter. The baseband input signals to this transmitter are digital signals indicative of a signal having an amplitude and a phase, and thus are in a polar coordinate form. Typically, the amplitude and phase values for a given symbol correspond to a point in a signal constellation, such as used in quadrature amplitude modulation (QAM). In this conventional arrangement, modulation of the baseband polar signals for multi-mode transmission involves the separate modulation of the baseband signals for each of the transmission modes. In this example, where GSM and WCDMA modes are to be generated, the in-phase and quadrature (I, Q) input digital signals are applied, for each symbol, to corresponding in-phase and quadrature modulators 4W, 4G, respectively. Modulators 4W, 4G generate in-phase and quadrature (I, Q) analog signals corresponding to the received input baseband symbols, and according to the particular constellation for the corresponding transmission mode.
The I and Q output signals from modulators 4W, 4G are then applied to corresponding mixers 6W, 6G to generate the output signals, directly upconverted to the appropriate carrier frequencies. In the case of mixer 6W for generating the WCDMA signals, the corresponding channel selection is made by way of phase-locked loop 8, to generate the carrier frequencies for each of the subchannels to be transmitted. The WCDMA signals generated by mixer 6W is an amplitude and phase modulated signal over a spread spectrum. The GSM signals generated by mixer 6G is effectively a phase-modulated signal, such as a Gaussian-Minimum-Shift-Keyed (GMSK) signal at the desired carrier frequency.
The conventional multi-mode transmitter of FIG. 1 is effective to generate multiple transmission mode signals from the input baseband signal. However, according to this architecture, each transmission mode has its own amplitude and phase modulator circuitry, because of the disparity among the various modulation schemes. This multiplication of circuitry of course results in relatively costly transceivers.
By way of further background, U.S. Pat. No. 6,047,029 describes a phase-locked loop modulator in which the data signal is applied to a ΣΔ (or Σ-Δ) modulator, the output of which controls a frequency divider in a phase-locked loop. The output of the phase-locked loop is a phase-modulated RF signal.
By way of still further background, U.S. Pat. No. 6,008,073 describes a phase-locked loop modulator in which digital modulation compensation for the effects of the low-pass loop filter is carried out. According to this approach, a digital pre-filter enhances higher frequencies of the modulating input signal, beyond the cutoff frequency of the loop filter, to compensate for the attenuation of these frequencies by the loop filter.